1. Field of the Invention
This invention relates to liquid level sensors using optical sensing of light through a quartz rod.
2. Prior Art
Quartz rod liquid-level sensors are commonly used to measure the level of a chemical liquid in a bubbler chamber. A carrier gas is bubbled through the chemical liquid in the bubbler chamber. This vaporizes the chemical liquid and the vapor is contained within the bubbler chamber in the space above the surface of the liquid chemical. The level of the liquid chemical within the bubbler chamber is sensed using a cylindrical quartz rod. The cylindrical quartz rod is inserted through an adapter mounted to the top cap of the bubbler chamber so that the bottom end of the quartz rod is near the top surface of the chemical liquid. The bottom end of the quartz rod is formed into a polished conical-shaped surface. A light beam from a light-emitting diode LED transmitter is projected through the top end of the quartz rod and down through the quartz rod to the conical-shaped bottom surface of the quartz rod. If the conical-shaped surface is not immersed in the chemical liquid, light is reflected back up the quartz rod to a detector located next to the laser diode transmitter. Conversely, if the conical-shaped surface is immersed in the chemical liquid, light passes through the conical end into the liquid chemical and is not reflected back through the quarts rod to the laser diode detector. The LED transmitter and the detector are both held within a plug assembly.
A prior-art plug assembly includes a locking nut which is threaded onto the adapter for mounting the quartz rod. The plug assembly forms an integral part o f the sealing mechanism for the quartz rod. Consequently, if it is desired to remove the plug assembly to service the LED transmitter and detector, the vapor seal for the bubbler chamber is broken, permitting contaminants to enter the bubbler system. These contaminants ultimately may be deposited on a semiconductor wafer being processed using chemical vapor from the bubbler system. If the system seal is broken, an operator may also be exposed to a hazardous chemical material.